The present invention relates to an optical disk apparatus, or more in particular to an erasable optical disk apparatus in which a laser beam approximately 1 .mu.m in diameter is irradiated on an optical recording medium thereby to record and reproduce a signal with high density and the signal thus recorded is erased by laser beam irradiation thereby to make possible repetitive signal recording and reproduction.
In an optical disk apparatus, signal recording is effected by modulating the intensity of the laser beam irradiated on the optical disk, by the signal recorded. The reproduction of the signal, on the other hand, is accomplished by irradiating a laser beam of a predetermined intensity on a signal recording section of the optical disk and thereby detecting variations of a reflected or transmitted light.
An example of the above-mentioned optical disk apparatus is shown in FIG. 6. In FIG. 6, reference numeral 101 designates a recording-reproducing semiconductor laser for generating a light beam of wavelength .lambda..sub.1, and an output light beam thereof is shown by l. Numeral 102 designates a condensing lens for condensing the output light of the semiconductor laser to make a substantially parallel light beam. Numeral 10 designates a light beam combiner for transmitting light of wavelength .lambda..sub.1 and reflecting light of wavelength .lambda..sub.2. Numeral 106 designates a beam splitter, and numeral 107 a reflection mirror. Numeral 108 designates a focus lens for focusing the light beam l to produce a light spot L on a guide track 51 of the optical disk. Numeral 109 designates an actuator for performing the well-known focusing control.
Numeral 103 designates a semiconductor laser for generating a light beam m of wavelength .lambda..sub.2, and numeral 104 a condensing lens for converting the light beam m into a substantially parallel light beam with an elliptical section. The light beam m is reflected on the beam combiner 105, and an elliptical light spot M with the longer diameter thereof directed longitudinally of the guide track is formed on the same guide track 51 as the light spot L as shown in FIG. 7.
In FIG. 6, the light reflected on the optical disk is applied through the focus lens 108 and the mirror 107 to the beam splitter 106, where the light path is changed and the light beam enters a filter 111. The filter 111 is adapted to pass the light l of the wavelength .lambda..sub.1 but not the light m of wavelength .lambda..sub.2. Numeral 112 designates a single lens for reducing the reflected light l. Numeral 113 designates a reflection mirror for reflecting a half of the light while passing the other half therethrough to a light detector 115.
Numeral 114 designates a half-split photo diode for detecting a focus error signal, which detects movement of the light l.sub.1 and produces a focus error signal. Numeral 115 designates a half-split photo diode for detecting the tracking error signal by the light beam l.sub.2.
Numeral 116 designates an erase laser drive circuit for driving the semiconductor laser 103 and controlling the intensity of the light spot M with the signal applied to the terminal Q.
Numeral 117 designates a record laser drive circuit for driving the semiconductor laser 101 and controlling the intensity of the light spot L with the signal applied to the terminal P.
FIG. 7 shows the two light spots irradiated on the guide track 51 of the disk shown in FIG. 6. The circular light spot L and the oval light spot M are arranged on the same guide track 51.
In this apparatus, in the event that the position of the laser source 103 mounted with mechanical accuracy is displaced by an operating temperature, environmental temperature or a shock, the erase function may be adversely affected with the erase light spot M displaced from the center of the guide track 51 carrying the light spot L causing failure to erase the data in the track involved fully or the undesirable erasure of the data in a neighbouring track. In an alternative case where the erase light spot M is subjected to tracking control, in contrast, the light spots L and M in FIG. 5 may follow different tracks, erasing the valuable data not to be erased in the neighbouring track.